The preparation of plant-protective agents poses two kinds of problems:
1) the preparation of an active ingredient possessing the desired biological activity; and
2) the formulation of the active ingredient in a composition providing easy handling for the user.
Among the usual formulations of plant-protective agents, the wettable powders (WP-s) and emulsifiable concentrates (EC-s) are most important and represent nearly 90% of the plant-protective agents available commercially at present. Being liquids, the EC compositions are easier to handle, their portioning can be handled by a simple volumetric measurement and, from the viewpoint of labor-safety it is advantageous that during mixing the release of the health-damaging materials into the air is not a problem (which is practically inevitable in the course of e.g. transferring powders from bags to mixing tanks). In addition, the biological activity of the EC compositions is higher than that of the WP compositions. On the basis of the obvious advantages of EC compositions, it could be expected that these formulations would play a predominant role on the market. Actually however, the number of WP compositions is somewhat higher than that of the EC-s. The reason for this is that an EC composition can be prepared only from an active ingredient which is liquid or, when a solvent can be found in which the active ingredient can be dissolved to give a solution of 10 to 85% concentration (depending on the usual concentrations of the application) without any risk of the interim alteration of the active ingredient. An other drawback of the EC compositions arises from their high solvent content whereby they are inflammable and explosive and the environment is polluted. The drawback of the EC compositions can be diminished by formulating the active ingredient in an emulsifiable microemulsion concentrate.
Microemulsion is a colloidal system which, in a first approach differs from a true emulsion in the dimension of its particles which are smaller by an order of magnitude than those of a true emulsion. According to the general definition, this system contains surface active agents and two immiscible liquids, one of them is usually water, though, in principle, it is also possible to prepare a water-free microemulsion by using an other solvent.
The surfactant may be the mixture of even 6 to 8 tensides and additionally, it may contain alcohols or amines of medium chain length as auxillary surfactants (co-surfactants). A peculiarity of this complicated system lies in its thermodynamical stability: it is formed spontaneously from the appropriate components without the input of an outer energy. The direct reason for this formation is that the interfacial tension between the two phases is very small, it approaches zero and transiently it even becomes negative.
In the outer appearance, a microemulsion is a transparent solution as a consequence of the very small size of the disperse particles. The size of the monodisperse spheres varies between 0.01 and 0.2 .mu.m. Although this system is transparent, all monochromatic lights such as electron or neutron radiation or X rays are scattered by the system whereby the determination of the particle size is also rendered possible.
Being a peculiar colloidal system, a microemulsion may prominently be used for formulating plant-protective agents on the basis of its above characteristics.
An additional advantage of the microemulsion consists in that it contains a lower amount of organic solvent than an EC composition which is preferably from the viewpoint of both environment protection and the danger of fire and explosion. Microemulsions are preferably used in the cases of ULV applications.
It is characteristic of the known microemulsion EC compositions that, in addition to the active ingredient they contain a combination of nonionic and anionic surface active agents as well as a high amount of a co-surfactant (capillary active substance).
In DE-OS Nos. 3,236,240 and 3,235,612 as well as in U.S. patent specification No. 4,469,675 microemulsion formulations are described which contain mixtures of: an alkylphenol polyglycol ether with calcium dodecylbenzenesulfonate; alkylphenol polyglycol ether with triethanolamine dodecylbenzenesulfonate; or polyoxyethylene-distyrene phenyl ether with calcium dodecylbenzenesulfonate and polyethyleneoxide sorbitol monostearate as combinations of nonionic and anionic surface active agents. Further on, each formulation contains a co-surfactant; most of the examples relate to cyclohexanone. The compositions reported in the above-cited United States patent specification always contain polyvinylalcohol, too.
Since the esters of thiophosphoric acid are decomposed even in weakly alkaline media, the anionic surface active agents commonly used for microemulsion compositions cannot be considered for the formulation thereof. In U.S. patent specification No. 4,304,587 an EC composition is described which contains an organic phosphoric acid ester dissolved in a mineral oil, in petroleum solvents, chlorinated hydrocarbons, alcohols, glycols, ethers, esters, ketones or their mixtures by using an anionic phosphate ester and an alkylphenol polyglycol ether nonionic tenside as emulsifying agent.
In GB-PS NO. 2,071,496 EC compositions are reported which contain organic phosphoric or thiophosphoric acid esters as active ingredient; these compositions contain 1 to 30% by weight of active ingredient as well as 20 to 70% by weight of a nonionic emulsifier and 20 to 60% by weight of a mineral oil or a softening agent. In the examples of this patent specification, nonylphenyl polyglycol ether (EO=10) is used as a nonionic emulsifying agent. When this EC composition is poured onto water, a true emulsion is formed spontaneously.
The problems arising from the formulation of phosphoric and thiophosphoric acid esters are avoided by the solution described in GB-PS No. 2,049,425. No effort is made to prepare an EC composition, but a concentrated aqueous true emulsion is prepared which is stabilized by a polymer (polyvinyl alcohol with a polymerization degree below 1500, tragacantha gum, guar gum, alginates, methyl-cellulose, polyacrylic acid and the like) dissolved in water. The droplets of the emulsion are rather large (1 to 50 .mu.m according to the examples) however, because of the high viscosity of the aqueous medium, no sedimentation or creaming has to be considered during the storage. A drawback of this composition is that it can be stored for only six months instead of the usual two years: furthermore, owing to the high viscosity, the composition is more difficult to dissolve than are the usual EC compositions.
An interesting way of formulating phosphoric and thiophosphoric acid esters is described in GB-PS No. 2,050,170, according to which the EC composition contains 10 to 99.5% of an organic acid or a mixture of organic acids. In this composition, the primary role of the tensides, preferably anionic tensides (polyoxyethylenated sorbitol oleate, alkylphenol polyglycol ether and the like) is to increase the wettability and not to stabilize the emulsion formed.
According to HU-PS No. 169,435, nonionic (alkylaryl polyglycol ether) and anionic (alkylbenzenesulfonate) surface active agents are used in a total amount of 10% together with a suitable co-surfactant (cyclohexanone, dimethylformamide and the like) for formulating O-phenyl-thiono-thiolphosphoric acid esters into an EC composition.
On the basis of the prior art described above it can be stated that no microemulsion composition is known which contains an organic phosphoric or thiophosphoric acid ester as active ingredient; furthermore, the formulations developed for microemulsion compositions cannot be adapted for phosphoric or thiophosphoric esters since these compositions contain basic components in amounts sufficient to start the chemical decomposition of the ester or to rapidly decrease the active ingredient during storage.